Heh heh, typically, the 400 2.8 lenses are heavier than the 500 f4 and only a tad lighter than the 600 f4, so if Sony's future offerings hold true then one could expect their version of the 600 f4 to be only marginally heavier than this new lens. That would be a game changer for many.The schematic drawings of the FE 500/4 and FE 600/4 suggest you'd better keep that credit card warmed up
I plan to keep this until the new lenses are announced and then possibly sell this for the 600 - or maybe not. A supertele with a fast f2.8 aperture is a very special beast.